[core] debounce current-time reminders by elapsed time (#29659)

## Summary
- rename `reminder_interval_model_requests` to
`reminder_interval_seconds`
- read the configured time provider before every model request and
inject a reminder only after the configured number of seconds has
elapsed
- preserve immediate first delivery and forced delivery after compaction
changes the context window

## Tests
- `just test -p codex-core current_time_reminder`

codex-rs/features/src/feature_configs.rs

@@ -144,7 +144,7 @@ pub struct CurrentTimeReminderConfigToml {
     pub enabled: Option<bool>,
     #[serde(skip_serializing_if = "Option::is_none")]
     #[schemars(range(min = 1))]
-    pub reminder_interval_model_requests: Option<u64>,
+    pub reminder_interval_seconds: Option<u64>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub clock_source: Option<CurrentTimeSource>,
 }